Fibers with micrometer and nanometer scale diameters and complex architectures have been of significant interest in recent years owing to their broad scope of applications in diverse fields like regenerative medicine, optoelectronics, sensor technology, protective clothing, filtration, catalysis, etc. Among fiber spinning techniques, electrospinning, melt blowing, and rotary jet spinning are currently popular for their capabilities of producing very thin fibers ranging from tens of nanometers to a few micrometers.
Electrospinning typically involves application of a strong electric field (usually from about 10 to about 20 kV) to a polymer solution (up to 95% solvent by weight) that is ejected out of a syringe. At a critical voltage, when the electrostatic repulsive forces in the solution subdue the surface tension forces, a jet of polymer solution is driven towards a grounded collector. Rapid evaporation of solvent in the air leaves behind solid polymer fibers on the collector. As the charged jet travels towards the collector it often experiences chaotic whipping motion and, under some circumstances, various instabilities. The whipping motion is believed to amplify the stretch ratio, defined as the initial fiber diameter divided by its final diameter, resulting in fine fibers (Shin, et al., Appl Phys Lett, 78:1149-1151, 2011; Reneker, et al., J Appl Phys 87:4531-4547, 2000). Though the technique is commonly used in research laboratories, it involves use of copious amounts of solvent (e.g., 80-95 wt %) and is plagued with severe environmental and economic challenges including solvent recycling/recovery, toxicity of solvents, and a lower mass throughput due to solvent evaporation (Zhou, et al., Polymer 47:7497-7505, 2006). Alternatively, one can electrospin fibers from polymer melts instead of solutions. Nevertheless, processing constraints due to the high viscosity and low conductivity of polymer melts and the need for high temperature equipment capabilities affects the commercial viability.
Melt blowing involves extrusion of molten polymer through a nozzle and further stretching the continuous filaments with jets of hot air to yield very thin fibers often exceeding 1-2 μm in diameter. Under special processing conditions, it has been recently shown that fibers below 500 nm diameter could be generated from a variety of polymers using melt blowing (Ellison, et al., Polymer 48:3306-3316, 2007; Tan, et al., J Non-Newton Fluid Mech 165:892-900, 2010). Since this process does not require any solvent, it appears to be environmentally benign. However, it requires significant thermal energy both for melting the polymer and for generating hot air jets with high flow rates to entrain the molten fiber and attenuate it to finer fiber.
A spinning technique called rotary jet or force spinning has recently been developed. This technique involves spinning a polymer solution or polymer melt through a rotating nozzle and relies on centrifugal force to draw the fibers. This process is believed to have a much higher production rate of fibers as compared to that of electrospinning and doesn't require high electric field. However the process still requires the polymers to be dissolved in solvents or heated to a melt prior to fiber spinning.
What is needed are methods of making fibers that are environmentally friendly and energy efficient. Further, methods of making fibers that are nearly free of defects is also desirable. The subject matter disclosed herein addresses these and other needs.